1. Field of the Invention
This invention relates to a method of making uniformly-sized polymeric particles and, more particularly, to a method of making polymeric particles having a very low population of undersized particles.
2. Description of Related Art
Polymeric particles find application as photographic matte beads, electrophotographic toner particles, thermal print spacer beads, polymeric microvoiding agents, grinding media, spreading layers, particle size standards, filter challenge media, ion-exchange resin packing media, Merrifield synthesis supports and the like. Such particles can be made by a variety of in situ polymerization techniques such as suspension polymerization or limited coalescence polymerization as well as by crushing or grinding of solid polymer samples, spray drying, evaporation of solvent from a dispersion of a polymer solution, etc. In order to obtain a narrow particle size distribution of useful product, all of these techniques require extensive sorting and classification operations that are costly, time-consuming and cause substantial reductions in yield. Microsuspension polymerization is frequently used to provide easily processable polymeric products in high yield for the above applications. However, even with microsuspension polymerization, the particle size distribution of polymer particles is not ideal. In particular, it is commonly observed that not only is a substantial population of undersized particles formed, but also appreciable amounts of coagulum, resulting in a loss of yield of useful product. The presence of these materials introduces the necessity in addition for complex processing steps in manufacturing (e.g., filtration, sieving, cyclone separation, and the like). An object of this invention is to provide an improvement on the microsuspension polymerization process which allows the production of polymer particles with much reduced populations of undersized particles.
Microsuspension polymerization is a term referring to a specific kind of suspension polymerization process. "Suspension polymerization" refers to a process in which a polymerizable liquid is dispersed as droplets in a continuous aqueous medium and polymerized under continuous agitation. Normally, this process is carried out in the presence of a "granulating agent", such as a lyophilic polymer (starch, natural gums, polyvinyl alcohol or the like) or an insoluble fine powder such as calcium phosphate. These granulating agents help to obtain a dispersion of droplets of the polymerizable liquid but do not provide sufficient stabilization of the dispersion so that the dispersed droplets are stable in the absence of agitation. Therefore, in this method, it is necessary to carry out the polymerization under continuous high-energy mechanical agitation, since otherwise extensive coalescence of the droplets will occur, with separation of a bulk phase of the water immiscible, polymerizable material or the formation of large amounts of coagulum. Because this process depends on the details of the shear field in the reactor, and on the changing viscosity of the polymerizing dispersed phase, it is difficult to control reproducibly, is not readily scalable, and gives broad particle size distributions (PSDs).
Microsuspension polymerization also refers to a process in which the water-immiscible polymerizable liquid is dispersed in an aqueous medium. In this process, as in suspension polymerization, the water insoluble monomer is dispersed in the presence of a dispersion stabilizer or granulating agent to the desired size by using a mechanical shearing device such as on agitator, a high pressure homogenizer, colloid mill, ultrasonic horn or the like. In contrast to simple suspension polymerization, however, in microsuspension polymerization, the polymerization can then be carried out with no or minimal stirring (only enough to prevent creaming and provide good thermal transfer). Various dispersion stabilizers or granulating agents are well-known in the art (for example, surfactants such as sodium dodecyl sulfate or sodium dioctylsulfosuccinate, and hydrophilic polymers, for example polyvinyl alcohol, gelatin, methyl cellulose, methylhydroxypropyl cellulose, ethyl cellulose, sodium salt of carboxymethyl cellulose, polyacrylic acid and salts thereof, starch, gum, alginic acid salts, zein, casein, etc.). In some cases, granulating agents useful for suspension polymerization are also useful for microsuspension polymerization. Which process occurs is a function of the nature of the oil phase; that is, whether the dispersion is stable in the absence of mechanical agitation or whether it will coalesce before or during the polymerization process. Suspension polymerization is used to provide easily filterable polymer products, but these products are generally of ill-defined particle size and size distribution, usually of between 50-1000 micrometers. Microsuspension polymerization can be used to provide products with mean particle sizes less than 20 micrometers. However, a disadvantage of the microsuspension method is that it is difficult or impossible to avoid the formation of a significant amount of polymer in the form of latex particles (of size less than 1 micrometer) at the same time. In an ideal microsuspension polymerization, the particle size distribution of the product polymer would match that of the original monomer dispersion; that is, the distribution of the particles formed during the dispersion step. The latex is presumed to be formed by diffusion of monomer from the large droplets to micellar structures in the aqueous phase. This process is well-known as conventional emulsion polymerization (latex polymerization) but is an unwelcome side reaction in microsuspension polymerization.